Light emitting elements such as LEDs are used not only as display elements themselves but also as backlight sources of a liquid crystal display (LCD). The number of light emitting elements used depends on the form of the display and the amount of light required for the display.
FIG. 3 shows the appearance of a typical electronic apparatus implemented in the form of a cellular phone utilizing LEDs as its light emitting elements. The cellular phone shown in FIG. 3(a) can be folded up as shown in FIG. 3(b).
As shown in FIG. 3, the cellular phone has an antenna 1, a large main display section 2 having a large display area, and a control section 3. The cellular phone also has a sub-display section 4 for displaying, for example, receipt of a phone call and e-mail, and for displaying date and hour, while it is folded up. This sub-display section 4 may have a small display area. These display sections 2 and 4 are constituted of LCDs utilizing, for example, white LEDs as backlight sources. The numbers of the LEDs backlighting the display sections 2 and 4 depend on the area of the respective display sections 2 and 4.
FIG. 4 illustrates a conventional circuit for driving LEDs of the cellular phone as shown in FIG. 3. The circuit includes a display device 40 utilizing LEDs and a drive device 30 for driving the display device 40.
The display device 40 has a series of two serially connected LEDs 41 and 42 for the sub-display section 4, and another series connection of four LEDs 43, 44, 45, and 46 providing enhanced intensity of light to the main display section 2.
On the other hand, the drive device 30 includes a step-up type switching power supply circuit 31 for stepping up a power supply voltage Vdd (typically 4V) of a lithium battery for example to a higher step-up output voltage Vhh. The step-up voltage Vhh is set to 18V, since each of white and blue LEDs requires about 4V for emission of light. This step-up voltage Vhh is applied to the LEDs 41-46 through the pin P31 of the drive device 30 and through the pin P41 of the display device 40. The drive device 30 also includes drivers 32 and 33 which are usually implemented as constant-current drivers. Hence, the drivers 32 and 33 supplies constant currents to the LEDs when they are turned ON, and shut down the currents when they are turned OFF, irrespective of the number of LEDs connected in series. The drivers 32 and 33 are turned ON or OFF in accordance with respective display instruction signals to control the operation of the LEDs 41-46.
In this conventional drive device 30 for LEDs, however, it is necessary to generate a sufficiently high voltage in accord with the numbers of the serial LEDs in the respective series by the power supply circuit 31. Consequently, the voltages of the pins P32 and P33 rise to the high step-up voltage Vhh when the drivers 32 and 33 are turned OFF. As a consequence, even though the power supply voltage Vdd is low, ICs designed to operate at an ordinary power supply voltage Vdd cannot be used in the drive device 30, unless the drive device 30 is designed to withstand the high step-up voltage Vhh.
Further, when the LEDs in the respective series are different in number, the constant-current driver 32 having less number of LEDs will consume extremely large power while driving the LEDs. Hence, in order to make the constant-current driver 32 withstand such large power consumption, the driver must be large-sized. Moreover, such large power consumption will result in quick consumption of available battery power.
It is therefore an object of the invention to provide a drive device for driving serially connected light emitting elements (hereinafter referred to as light emitting element series), capable of always impressing voltages lower than the supply voltage to the pins to which the light emitting elements series are connected, irrespective of the number of the light emitting elements in the series, thereby enabling use of low-voltage ICs and minimization of the power consumption by the drivers. It is another object of the invention to provide electronic apparatuses equipped with light emitting elements driven by such drive device.